The present invention refers to a process for making an expansion screw for orthodontics and the orthodontic screw thus made.
Shown in FIGS. 1 and 2 of the attached drawings is an example of expansion screw of known type for orthodontic use. The screw comprises, substantially, a bearing structure with two main bodies (1, 2) provided with arms (8) which allow it to be associated to orthodontic bands (or other component of different conformation and/or function) to complete an orthodontic apparatus to be used for expanding the palate by suitably displacing the two bodies (1, 2) apart. The said bodies (1, 2) exhibit a central bore (30) with inner left-hand threading by one side and a right-hand threading by the opposite side. The two bodies are disposed in facing relationship and connected to each other by a double screw (3) having a central portion (6), with a plurality of driving bores (7), which is intended to result in an intermediate position between the same bodies (1, 2). The adjustment of the operating condition of the apparatus, during the orthodontic treatment, is made by means of a suitable tool inserted within the bores (7) of said central portion. The said screw (3) is double, that is, it exhibits two threaded spindles, extending from opposite sides with respect to the central portion (6), each spindle is intended for engagement with the central threaded bore of one of said bodies (1, 2), so as to causes them to move away from, or close to each other depending on the [direction of] rotation into which the central portion (6) is driven during said adjustment. Moreover, the said main bodies (1, 2) exhibit, in addition to the threaded central bore, two more bores (5) parallel to the central one and extending from opposite sides with respect to the axis thereof. The additional bores (5), without threading, have two cylindrical guide pins (4) going therethrough.
When using the expansion screws of a so-called xe2x80x9cfreexe2x80x9d type, because they are not immersed in the resin of a palate plate, a clearance is provided between the pins (4) and the bores (5), as necessary for the sliding of same pins and as a consequence of the work tolerances. In order to limit the negative effects of such clearance, the coupling between the central screw (3) and the threaded bores (30) which accomodate it is made by friction. In practice, chemicals of so-called xe2x80x9cthread-brakingxe2x80x9d type are used to create a friction between the threaded spindles of the central screw and the threads mating therein. In this way, unwanted rotating movements of the central screw, likely to endanger the success of the orthodontic treatment, are counteracted.
A drawback related to this production technique stems from the fact that a thus treated screw cannot be subjected afterwards to any thermal or chemical treatments (such as those for cleaning or welding the bands) which would imply the suppression of the involved products with consequent suppression of the braking function they perform. Besides, the thread-braking chemicals may result in etching by substances present in the oral cavity and be dissolved.
A further drawback of the expansion screws of known type derives from the maximum extent of the expansion which cannot pass the physical limits of the length of the guide pins. Moreover, when positioning the apparatus in its maximum expansion configuration, the same apparatus may be somewhat unstable, since the pins (4) are almost entirely withdrawn from the relevant bores (5) (see FIG. 2).
A further drawback of the expansion screws of known type derives from the maximum extent of the expansion which cannot pass the physical limits of the length of the guide pins. Moreover, when positioning the apparatus in its maximum expansion configuration, the same apparatus may result somewhat unstable, since the pins (4) are almost entirely withdrawn from the relevant bores (5) (see FIG. 2).
A further (known) technique provides for deforming the threading of the nut screw which accomodates the threaded spindles of the central screw. However, in practice, it is impossible to ensure the same deformation all the time, so that, in some cases, a seizure effect may occur in the system after a brief period of use or, in other cases, the friction may result insufficient in relation to the work tolerances of the mating elements.
The main object of the present invention is to overcome the said drawbacks.
This result has been achieved, according to the invention, by providing an operating process and an orthodontic expansion screw having the characteristics indicated in the independent claims. Further characteristics being set forth in the dependent claims.
The advantages deriving from the present invention lie essentially in the fact that it is possible to ensure optimal stability and rigidity of the expansion screw when this is in use; that there is obtained a relatively wide expansion in relation to the overall dimensions of the screw; that the screw is safely retained in spite of its limited dimensions and expansion extent obtained (that is, with equal expansion, there is obtained a minor bulkiness of the guides and a larger mating surface, with respect to the known solutions); that the present screw can be subjected to physical and chemical treatments of sterilization or others, while maintaining its characteristics even after a prolonged service life.